Promoting high-voltage stability through local lattice distortion of halide solid electrolytes

Stable solid electrolytes are essential to high-safety and high-energy-density lithium batteries, especially for applications with high-voltage cathodes. In such conditions, solid electrolytes may experience severe oxidation, decomposition, and deactivation during charging at high voltages, leading to inadequate cycling performance and even cell failure. Here, we address the high-voltage limitation of halide solid electrolytes by introducing local lattice distortion to confine the distribution of Cl−, which effectively curbs kinetics of their oxidation. The confinement is realized by substituting In with multiple elements in Li3InCl6 to give a high-entropy Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6. Meanwhile, the lattice distortion promotes longer Li-Cl bonds, facilitating favorable activation of Li+. Our results show that this high-entropy halide electrolyte boosts the cycle stability of all-solid-state battery by 250% improvement over 500 cycles. In particular, the cell provides a higher discharge capacity of 185 mAh g−1 by increasing the charge cut-off voltage to 4.6 V at a small current rate of 0.2 C, which is more challenging to electrolytes|cathode stability. These findings deepen our understanding of high-entropy materials, advancing their use in energy-related applications.


Itemized list of responses to the reviewers' report
(Black italics: Reviewers' report; Blue type: Reply to the Reviewer) Reviewer #1: The authors report about the successful synthesis of a chlorine-based high entropy solid electrolyte.They evaluated their local-and long-range crystal structure and electrochemical properties.Introducing high entropy in such class of materials seems to lead to better electrochemical stability and higher ionic conductivity.This study might pave the way for an extensive exploration of multielement substituted halide-based solid electrolytes.Therefore, the manuscript could potentially be accepted for publication in Nature Communications after a major revision.
Reply to the Reviewer: We highly appreciate the referee for his/her summaries and positive comments of our work.High-entropy halide electrolyte's combination of largely enhanced electrochemical stability and high ionic conductivity makes this work novel and attractive.Below are the point-by-point responses.
1.In the introduction section and ex situ evaluation of the SSB cells (line 309 following), the authors state several times that chloride oxidation is a severe issue for chlorinebased solid electrolytes if high potentials are reached.Please discuss this in more detail, as it seems highly unusual.What species might be formed and are there references (also reference XPS spectra which have observed similar contributions in the Cl XPS spectra)?
Reply to the Reviewer: The authors thank the reviewer for his/her important comments.As shown in Figure R1, we first calculated the electrochemical window of Li3InCl6 (LIC) based on Density Functional Theory (DFT) calculations using Material Project.The cyclic voltammetry (CV) was also used to ascertain the oxidation potential of chloride SSEs (Figure R2), which aligns with theoretical calculation result.Clearly, as we set the cut-off voltage close to the oxidation potential and the catalysis of cathode active material, LIC undergoes inevitable oxidation in practical batteries.This unavoidable oxidation would result in capacity decay during long-term cycling of solidstate cells with LIC (Figure R3).To clarify the oxidized compositions, X-ray photoelectron spectroscopy (XPS) analysis was conducted.As can be seen from Figure R4a, a higher binding energy of Cl 2p peak is observed after oxidation, which shows a similar result with previous studies (Figure R5) 1 .The resulting decomposition products include LiClO4, InClO etc. 2,3 In sharp contrast, the as-designed Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6(HE-LIC) induces lattice distortion due to the atoms of different radii.The lattice distortion leads to sluggish diffusion, primarily attributed to the coordinated diffusion of multiple elements, particularly chlorine, during the oxidation process.As a result, HE-LIC exhibited improved oxidation stability (Figure R2) and long-term cycling performance (Figure R3).This result is substantiated by the XPS analysis conducted after cycling (Figure R4b), which revealed a diminished signal of oxidation products.These findings confirm the considerable potential of high-entropy materials in enhancing the stability of all-solid-state batteries.Reply to the Reviewer: We thank the reviewer for his/her in-depth comments.
As the reviewer mentioned, we have conducted investigation on the equimolar composition at first, and we should include this optimization content in the revised manuscript.The equimolar Li2.8Y0.2Er0.2Yb0.2In0.2Zr0.2Cl6(HE-LIC-equimolar), exhibits similar XRD patterns as the non-equimolar Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6(HE-LIC), as depicted in Figure R6.The HE-LIC-equimolar exhibits a Li + conductivity of 0.930 mS cm -1 and an activation energy of 0.352 eV (Figure R7).Furthermore, we have discovered that by slightly adjusting the proportion of certain elements on this basis, we can further enhance the electrochemical performance.For instance, incorporating more quadrivalent Zr results in additional lithium vacancies 4 .With more In, it can exhibit enhanced ionic conductivity in heat treatment compared to Y, Er, and Yb 5,6 .Meanwhile, we have to ensure that the configurational entropy is not significantly compromised.Therefore, we reach the non-equimolar formula Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6.As expected, this composition demonstrates an ionic conductivity of 1.171 mS cm -1 and a lower activation energy (0.338 eV) compared to HE-LIC-equimolar.They both show enhanced oxidation stability (Figure R7d).Therefore, we selected HE-LIC as the primary focus of this study.
We have added the related information in the revised manuscript and supporting information accordingly.

Please give standard deviations for the refinement tables in the supp info (if freely refined, or was everything fixed?)
Reply to the Reviewer: By following the important suggestion, we have added the standard deviations for the revised refinement tables.5. Please give details for the refinement of the NPD data.Especially as some used isotopes/elements show strong neutron absorption cross section for neutrons, how was this issue treated during refining the diffraction data?
Reply to the Reviewer: We thank the reviewer for his/her questions.
For the powder neutron diffraction measurement, about 2 g powdery samples were put into the Ti-Zr null matrix alloy holders.Each sample was measured for ca. 3 hours at multiple physics instrument at China Spallation Neutron Source (CSNS).The background in the neutron pattern was slightly higher for the high entropy solid electrolyte which was due to the presence of Cl, Yb, Li, Er, and In ions as they have the absorption cross section of 33.5, 34.8, 70.5, 159, and 193.8 barns respectively.On the other hand, Zr, Y, contribute relatively low background as they have low absorption cross section of 0.185 and 1.28, respectively.The occupancies of Er and In were only 16 mol.%and 25 mol.% in the 4g sites (0, 0.3333, 0), therefore the general absorption of the sample was still acceptable for accurate structural analysis.The sample absorption parameter was calculated and manually input into the refinement process.
We have added the related information in the revised manuscript.

Institute of New Energy for Vehicles School of Materials Science and Engineering
Tongji University Shanghai 200092, China   E-mail: weiluo@tongji.edu.cnReviewer #2: The publication reports the enhancement of properties of Li electrolyte material based on Li3InCl6 through the incorporation of various dopants, resulting in the formation of a high-entropy material.However, the results obtained from described experiments do not seem to convincingly confirm the conclusions drawn by the authors of the work.Some issues within the study are outlined below.
Reply to the Reviewer: We thank the reviewer for the overall evaluations on our study.In response to the reviewer's concerns and make our discovery clearer, we have incorporated more experiments and analysis as below.
1.The authors conclude that the enhancement in properties stems from localized lattice distortions, which arise due to the formation of a high-entropy material.However, due to the nearly identical ionic radii and the degree of oxidation of all the dopants, one may have doubts whether the change results only from the introduction of several different dopants into the structure.A study previously reported in doi.org/10.1021/acs.chemmater.1c01348,showcased a similar enhancement in ionic conductivity by doping Li3InCl3 with zirconium, resulting in a conductivity increase to 1.2 mS/cm.The authors suggested that properties improvement was mainly due to an increased number of vacancies in the Li sublattice.This prior research seems to challenge the conclusions drawn in the present paper.
Reply to the Reviewer: We thank the reviewer for raising this question.
We agree with the reviewer that the introduction of Zr doping can enhance the ionic conductivity by creating more lithium vacancies.To address this query, we also synthesized Zr-doped LIC (Li2.6In0.6Zr0.4Cl6).The XRD analysis reveals that the crystal structure of Li2.6In0.6Zr0.4Cl6aligns with that of LIC (Figure R8).As shown in Figure R9, Li2.6In0.6Zr0.4Cl6exhibits an improved ionic conductivity (1.078 mS cm -1 ) and a reduced activation energy of 0.342 eV compared to LIC (0.849 mS cm -1 and 0.357 eV), respectively, which is consistent with the reported article mentioned by the reviewer.We also added the article as Ref. 14 in the revised manuscript.
In this work, we would like to simultaneously improve the ionic conductivity and the oxidation stability of chloride SEs by introducing the lattice distortion.The Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6(HE-LIC) exhibits an even higher ionic conductivity and lower activation energy than Li2.6In0.6Zr0.4Cl6.This suggests that the improvement of HE-LIC's performance is not solely attributed to the increase in lithium vacancies resulting from Zr doping.Rather, it is also influenced by a significant presence of cation disorder defect 7,8 .It has been proved that creating more configurational disorder within cation sites is an important strategy for promoting fast Li + migration in chloride SEs 9 .In our study, the cation disorder leads to the aggregated distribution of Cl -, and elongates the Li-Cl bond.This structure in turn inhibits the binding of the anion framework to Li + , and enables the favorable Li + conduction.Moreover, Li2.6In0.6Zr0.4Cl6exhibits an even lower oxidation potential than LIC, as revealed by the CV tests (Figure R9).This result indicates that Zr doping cannot enhance the high-voltage stability of halide solid electrolytes.Therefore, we found that the high-entropy halide solid electrolytes represent an innovative approach to enhance the electrochemical properties of halide solid electrolytes.

Institute of New Energy for Vehicles School of Materials Science and Engineering
Tongji University Shanghai 200092, China E-mail: weiluo@tongji.edu.cn

The authors of the work should show the results of the chemical composition analysis
of the tested samples.It should be ruled out whether the increase in the stability of the material is not caused by the introduction of an admixture of oxygen to the material, for example as a result of a long grinding time.The authors did not provide a detailed description of the sample preparation process, but conventional practice in this type of study involves utilizing ZrO2 vials and balls during grinding.Given the lengthy grinding duration employed in this study, the possibility of introducing extra ZrO2 into the material becomes significant, which would be consistent with the formation of an additional LiCl phase.The effect of improving the stability and conductivity in lithium electrolytes by oxygen substitution has been previously described in the literature for other materials.doi.org/10.1021/acs.chemmater.9b00505 Reply to the Reviewer: The authors appreciate the reviewer's meaningful comments.
We fully agree with the reviewer's concern that the possible introduction of oxygen doping will improve the performance of solid electrolytes, which has been reported in relevant articles.
In this study, we prepared the LIC and HE-LIC with the same ball milling time (30 h).We first investigate the LIC sample using X-ray photoelectron spectroscopy (XPS).As shown in Figure R10, LIC did not exhibit discernible Zr 3d signal, which means that the ball milling procedure would not introduce ZrO2 into the solid electrolytes.To further eliminate concerns about ZrO2 doping, we employed inductively coupled plasma optical emission spectrometer (ICP-OES) to study the HE-LIC (Table R1), which gives the chemical formula as Li2.797Y0.160Er0.159Yb0.162In0.254Zr0.250Cl6.002,almost the same as the ingredient proportion, indicating that no ZrO2-containing impurities are introduced during the ball milling process.
Besides, per the kind suggestions of the reviewer, we have described the synthesis process in detail in the revised manuscript as: "The preparation of all compounds was conducted under an argon (Ar) atmosphere.LiCl (99%, Aladdin), YCl3 (99.95%,Aladdin), ErCl3 (99.9%,Aladdin), YbCl3 (99.9%,Aladdin), InCl3 (99.99%,Aladdin) and ZrCl4 (99.9%,Aladdin) were used as received.For preparing high-entropy Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6(HE-LIC), about 2 g precursors were weighed according to the chemical formula and ground in a mortar evenly for 15 min.Then the precursors were subjected to ball milling in a ZrO2 pot with ZrO2 balls at 550 rpm for 30 h.The mass ratio of balls to precursors was 30:1, and the milling process involved alternating periods of 15 minutes of ball milling followed by 5 minutes of rest.Subsequently, the mixture was pelletized and annealed at 260 °C for 5 h with heating rate of 2 °C min -1 under Ar atmosphere.The pristine Li3InCl6 (LIC) and mid-entropy Li2.75Y0.5In0.25Zr0.25Cl6(ME-LIC) were prepared similarly.Argyrodite Li6PS5Cl (LPSCl) electrolytes were synthesized as the reported articles."wt.%) characterized as Pnma-type Li3YCl6.Furthermore, the neutron PDF analysis of the ME-LIC sample also confirmed this result (Figure R12).As illustrated in Figure R13, ME-LIC shows a Li + conductivity of 0.930 mS cm -1 , an activation energy of 0.341 eV and an electronic conductivity of 3.974 × 10 -9 S cm -1 . Furthermore, the CV test exhibits a comparable oxidation potential of 4.28 V as LIC (Figure R14).However, it should be noted that that the electrochemical properties of ME-LIC may not accurately reflect the performance of the target product, Li2.75Y0.5In0.25Zr0.25Cl6,due to the significant presence of this second phase.
We have added the relevant characterization results and discussion of ME-LIC in the revised supplementary information accordingly.

The authors did not describe the influence of on the stability towards Li. No justification was provided for the use of an additional LPSCl layer for the construction of the cells.
Reply to the Reviewer: We thanks for the reviewer's important questions.
To investigate the stability of halide solid electrolytes towards Li, we first performed CV tests on both LIC and HE-LIC in the voltage range of 0-3.0 V vs. Li + /Li. Figure R15 illustrates that both LIC and HE-LIC exhibit a weak cathodic peak at approximately 2.2 V.This peak corresponds to the initiation of a weak reduction process, specifically the conversion from In 3+ to In 2+ .Notably, when the voltage drops below 1.6 V, LIC undergoes a significant reduction reaction, whereas HE-LIC experiences reduction below 1.1 V. Furthermore, the reduction reaction current of HE-LIC is considerably lower than that of LIC, indicating that HE-LIC demonstrates improved reduction stability.
To further assess the reduction stability of LIC and HE-LIC, we assembled symmetric cells with Li-In anodes (Figure R16).The polarization voltage of the Li-In|LIC|Li-In symmetric cell exceeds 5 V, and the impedance exceeds 10000 Ω after just one cycle.HE-LIC exhibits improved cycling stability, as evidenced by a stable cycle lasting for 400 h.However, the polarization voltage remained above 0.8 V, and the impedance reached 4000 Ω after cycling, suggesting that HE-LIC is also unstable to Li-In.So, we have chosen Li6PS5Cl (LPSCl) to prevent the reduction of halide electrolytes.Meanwhile, LPSCl enable a relatively low interface impedance with halide SEs 10 .It is believed that the combination of stable SEI-forming LPSCl together with halide SEs as cathode electrolyte may be a suitable solution in practice.In LPSCl based Li-In symmetric cells, it achieved a consistent and stable cycle lasting for 400 h.Notably, LPSCl demonstrated a polarization voltage below 0.01 V and an impedance of only 10 Ω.Moreover, per the kind suggestions of the reviewer, the relevant content has been added to the revised Supplementary Information.(Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6).The authors elucidate that local lattice distortion in the HE-LIC structure leads to increased ionic conductivities and oxidation stability.The structural characterization and analysis of HE-LIC with highly complex MCl6 frameworks are carried out meticulously using neutron diffraction, TEM, EDX, STEM, and especially the calculation of configurational entropy.However, I have several doubtful points regarding the high-entropy solid electrolyte developed in this paper.
Reply to the Reviewer: We appreciate the reviewer for his/her overall summaries and comments of our work.In the subsequent sections, we provide responses to reviewer's questions.
1. Firstly, the HE-LIC proposed by the authors comprises expensive rare earth materials (Y, Er, Yb).Therefore, the authors need to provide insight into how to design and synthesize high-entropy electrolytes containing cheaper and more abundant elements such as Mn, Fe, Ti, or similar alternatives.
Reply to the Reviewer: We thank the reviewer for raising this question.
We totally agree with the reviewer that synthesizing solid electrolytes with cheaper and more abundant elements is meaningful.Thus, we substituted the rare earth elements Y, Er, and Yb with trivalent elements Al, Ti, and Fe to synthesize a new high-entropy (HE) electrolyte, Li2.75Al0.16Ti0.16Fe0.16In0.25Zr0.25Cl6.The XRD analysis, as depicted in Figure R17, reveals that this new compound also belongs to the monoclinic space group of C2/m.Then, we further measure the other properties of Li2.75Al0.16Ti0.16Fe0.16In0.25Zr0.25Cl6,which exhibits a Li + conductivity of 0.454 mS cm - 1 , an activation energy of 0.395 eV and an electronic conductivity of 3.633 × 10 -9 S cm - 1 (Figure R18).Obviously, the new HE electrolyte based on more abundant elements shows a poor Li + conduction ability compared to the original Li3InCl6 (0.849 mS cm -1 ) and the HE electrolyte based on rare earth elements (1.171 mS cm -1 ).
Although the HE electrolyte based on more abundant and cost-effective elements is attractive, its electrochemical performance is worse than rare elements based HE-LIC.We hope the reviewer could kindly agree with us that the high-entropy concept is useful for the design of new solid-state electrolytes (SSEs), and the development of highentropy SSEs and halide SSEs is still in their early stage.Further studies are undergoing in our lab to investigate the substitution of rare earth elements in halide SSEs.Reply to the Reviewer: The authors appreciate the reviewer's meaningful comment.
We fully agree with the reviewer's opinion that solely enhancing Li + conductivity and reducing activation energy may not suffice to significantly improve the performances of the battery.The increased ionic conductivity can reduce the impedance of the battery and enable the cathode to deliver a higher capacity.Moreover, the high-voltage stability of the HE-LIC is playing a more important role in influencing the cycling stabilities of batteries.Specifically, LIC is prone to oxidation side reactions when charged to high voltage.Previous studies have addressed these challenges by incorporating F doping, which unfortunately compromises Li + conductivities 14,15 .In this work, we adopt a highentropy strategy to enhance high-voltage stability without compromising ionic conductivities.Specifically, by implementing this strategy, the Li-In | LPSCl | HE-LIC | LCO cells achieve an enhancement in the initial Coulombic Efficiency from 94.9% to 97% (Figure R20a).Consequently, when subjected to long-term cycling conditions, the capacity retention is significantly improved from 71.8% to 88.9% (Figure R20b).

Figure R1 .
Figure R1.Thermodynamic equilibrium voltage profile and phase equilibria of Li3InCl6, which is derived from Density Functional Theory (DFT) calculations based on Material Project.

Figure R2 .
Figure R2.CV curves of LIC and HE-LIC within 2.7 and 5.0 V vs. Li + /Li.

Figure R3 .
Figure R3.Long-term cycling performance of solid-state cells using LIC or HE-LIC SSEs between a voltage window of 2.5-4.2V at 0.5 C.

Figure R9 .
Figure R9.Electrochemical properties of Li2.6In0.6Zr0.4Cl6,HE-LIC and LIC. a Typical Nyquist plots at room temperature, normalized for the pellet thickness and area.b Arrhenius conductivity plots.c Summary of electrochemical properties.d CV curves of Li2.6In0.6Zr0.4Cl6,compared with HE-LIC and LIC.

Figure
Figure R11.a X-ray diffraction patterns of LIC, ME-LIC and HE-LIC.b Neutron diffraction patterns and the corresponding refinements of ME-LIC.

Figure R13 .
Figure R13.Electrochemical properties of ME-LIC.a Typical Nyquist plots at room temperature, normalized for the pellet thickness and area.b Arrhenius conductivity plots.c DC polarization curves with an applied voltage of 1 V. d Summary of electrochemical properties.

Figure
Figure R20.a The initial cycle charge−discharge curves of cells tested at 0.1C.b Longterm cycling performance of the cells at 0.5 C.
By following the kind suggestion of the reviewer, we have added the related papers asRef.38,39, and 40in the revised manuscript.

Table 1 .
Structural refinement details from the neutron diffraction data for HE-LIC.

Table 3 .
Structural refinement details from the neutron diffraction data for LIC.

Table R1 .
Stoichiometry of HE-LIC according to the ICP-OES analysis.Atomic ratios of Li, Er, Yb, In and Zr have been normalized by Y content.
We have carried out related experiments and added more evidence to address the reviewer's concerns.The XRD and ND refining results of ME-LIC are shown in FigureR11.Obviously, ME-LIC is not a single phase, which consists of a major phase (63 wt.%) isostructural with Li3InCl6 (C2/m structure) and a distinct second phase (37